A magnetic recording medium, e.g., a hard disk, typically comprises a laminate of several layers, comprising a non-magnetic substrate, such as of Al—Mg alloy or a glass or glass-ceramic composite material, and formed sequentially on each side thereof, a polycrystalline underlayer, typically of chromium (Cr) or Cr-based alloy, a polycrystalline magnetic recording medium layer, e.g., of a cobalt (Co)-based alloy, a hard, abrasion-resistant, protective overcoat layer, typically containing carbon (C), and a lubricant topcoat.
In operation of the recording medium, the polycrystalline magnetic recording medium layer is locally magnetized by a write transducer, or write head, to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based on the bits of information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium layer, then the grains of the polycrystalline recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the polycrystalline recording medium can subsequently produce an electrical response in a read transducer, allowing the stored information to be read.
Thin film magnetic recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (CSS) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk, and stopping.
As a consequence of the above-described cyclic CSS-type operation, the surface of the disk or medium surface wears off due to the sliding contact if it has insufficient abrasion resistance or lubrication quality, resulting in breakage or damage if the medium surface wears off to a great extent, whereby operation of the disk drive for performing reading and reproducing operations becomes impossible. The protective overcoat layer is formed on the surface of the polycrystalline magnetic recording medium layer so as to protect the latter from friction and like effects due to the above-described sliding action of the magnetic head. Abrasion-resistant, carbon (C)-containing protective coatings have been utilized for this purpose, and are typically formed by sputtering of a carbon target in an argon (Ar) atmosphere. Such amorphous carbon (a-C)-containing protective overcoat layers formed by sputtering have relatively strong graphitic-type bonding, and therefore exhibit a low coefficient of friction in atmospheres containing water (H2O) vapor, which characteristic is peculiar to graphite. However, the a-C layers produced in such manner have very low hardness as compared with many ceramic materials such as are employed as slider materials of thin film heads, and thus are likely to suffer from wear due to contact therewith.
In recent years, therefore, carbon-based protective overcoat layers having diamond-like hardness properties (i.e., HV of about 1,000–5,000 kg/mm2) have been developed, and films of diamond-like carbon (DLC) having a high percentage of diamond-type C—C bonding have been utilized. Such DLC films exhibit a high degree of hardness due to their diamond-like sp bonding structure, and in addition, exhibit the excellent sliding properties characteristic of carbon, thus affording improved sliding resistance against sliders composed of high hardness materials. Such DLC films are generally obtained by DC or RF magnetron sputtering of a carbon target in a gas atmosphere comprising a mixture of Ar gas and a hydrocarbon gas, e.g., methane, or hydrogen gas. The thus-obtained films exhibit DLC properties when a fixed amount of hydrogen is incorporated therein. Incorporation of excessive amounts of hydrogen in the films leads to gradual softening, and thus the hydrogen content of the films must be carefully regulated.
Amorphous, hydrogenated carbon films (referred to herein as a-C:H films) obtained by sputtering of carbon targets in an Ar+H2 gas mixture exhibiting diamond-like properties have also been developed for improving the tribological performance of disk drives; however, the electrical insulating properties of such type films lead to undesirable electrical charge build-up or accumulation during hard disk operation which can result in contamination, glide noise, etc. In order to solve this problem without sacrifice or diminution of the advantageous mechanical properties of such a-C:H films, attempts have been made to form bi-layer structures including a lower C:H overcoat layer and an upper, nitrogen-containing C:H overcoat layer, or to dope or otherwise incorporate nitrogen (N) atoms into the surface of a C:H protective overcoat, in order to decrease the electrical resistivity thereof and/or to provide increased bonding of the lubricant topcoat layer to the protective overcoat layer.
However, the continuous increase in areal recording density of magnetic recording media requires read/write transducers operating at a commensurately lower flying height. Therefore, further reduction of the thickness of the carbon-based protective overcoat layer without adverse consequences is desirable. Conventional sputtered a-C:H materials are difficult to uniformly deposit and generally do not function satisfactorily at a thickness of about 30 Å or less. Specifically, conventional sputtered a-C:H films of about 30 Å thickness fail to provide adequate protection against corrosion of the underlying magnetic layer(s), particularly Co-containing ferromagnetic layers, when under environments of high temperature and humidity, and the resulting corrosion product(s) frequently are disadvantageously transferred to the transducer heads, often leading to failure of the disk drive.
The use of alternative deposition techniques for developing thinner, harder, and more dense C:H layers having the requisite mechanical and tribological properties has been studied, such as plasma enhanced chemical vapor deposition (PECVD), ion beam deposition (IBD), and filtered cathodic arc deposition (FCAD) techniques. For example, the IBD method can be utilized for forming high carbon density, hydrogenated carbon films (referred to herein as I—C:H films) that exhibit superior tribological performance at thicknesses below about 100 Å.
As indicated supra, the continuous increase in areal recording density of disk-type recording media has necessitated development of even thinner carbon-based protective overcoat layers than heretofore utilized, e.g., <˜30 Å thick, which thin overcoat layers are still required to protect the media from both tribological (i.e., mechanical) and chemical degradation. It is considered that overcoat layers with an increased density of carbon (C) atoms vis-à-vis conventional carbon-based protective overcoat materials are required for such high areal recording density media.
Filtered cathodic arc deposition (FCAD) is an attractive candidate for providing carbon-based protective overcoat layers with the requisite high carbon atom density and can be implemented in a cost-effective manner. For example, as shown in graphical form in FIG. 1, DLC protective overcoat layers comprised of tetrahedral amorphous carbon (ta-C) produced by FCAD are of significantly greater mass density than other types or forms of DLC, such as a-C:H formed by sputtering and I—C:H formed by ion beam deposition (IBD). However, since the ta-C films produced by FCAD do not contain either hydrogen (H) or nitrogen (N) atoms, and are highly sp bonded, their population of dangling bonds is very high, resulting in very high surface energy. The latter characteristic in turn disadvantageously engenders issues related to deleterious interactions between the carbon atoms and lubricant topcoat molecules, e.g., resulting in a higher than expected (or desired) bonded lubricant ratio leading to poor durability of the head-disk interfaces.
Accordingly, there exists a need for an improved hard, abrasion and corrosion-resistant, high carbon density ta-C protective overcoat material such as is formed by FCAD, which is particularly suitable for use as ultra-thin (i.e., <30 Å thick) protective overcoat layers in high areal density magnetic recording media utilized with read/write transducers operating at extremely low flying heights, and a method for manufacturing same, which method is simple, cost-effective, and fully compatible with the productivity and throughput requirements of automated manufacturing technology.
The present invention fully addresses and solves the above-described problems attendant upon the formation of ultra-thin, abrasion and corrosion-resistant, high carbon density ta-C protective overcoat layers formed by FCAD and suitable for use with high areal density magnetic recording media, such as are employed in hard drive applications, while maintaining full compatibility with all mechanical and electrical aspects of conventional disk drive technology. In addition, the present invention enjoys utility in the formation of ultra-thin, abrasion and corrosion-resistant protective overcoat layers required in the manufacture and use of thin film-based, ultra-high recording density magneto-optical (MO) data/information storage and retrieval media in disk form and utilizing conventional Winchester disk drive technology with laser/optical-based read/write transducers operating at flying heights on the order of a few micro-inches above the media surface.